1. Field of the Invention
The present invention relates to a lens for reading an original and used in an original reader, etc.
2. Description of the Related Art
An original reading system for scanning an original and reading an image of the original by an image pickup element such as a charge coupled device (CCD) is used in a facsimile, an image scanner, a digital copying machine, etc. A lens for reading the original is a lens for focusing and forming a reduced image of the original onto the image pickup element in such an original reading system.
With respect to the lens for reading an original, it is generally necessary to focus and form an image having a high contrast on an original face at a high spatial frequency until 4 line pair/mm so as to accurately read the original. Further, it is also necessary that a vignetting factor is equal to 100% until a peripheral portion of this image.
In general, a known lens for reading an original is constructed by a lens of a Gauss type, a frog type, a triplet type, a topogon type, etc.
The lens for reading an original preferably has a wide field angle to reduce a distance between an object and an image formed by this lens such that an original reader is made compact. Further, this lens preferably has a large aperture and a large brightness as much as possible to read the original at a high speed.
The general lens of a Gauss type, etc. are not necessarily constructed sufficiently in consideration of the wide field angle, the large aperture and the large brightness.
Namely, the lens of a Gauss type is suitable for the large aperture, but is not suitable for the wide field angle. The lenses of frog and triplet types have a small half field angle of about 20 degrees and a small brightness of about an F-number F/No.=4.5 to 5.6. The lens of a topogon type has a relatively wide field angle, but has a small half field angle of about 23 degrees.